1. Field of the Invention
The present invention relates to a method and related circuit for clock generation and recovery, and more particularly, to clock generation and recovery in an optical disk drive.
2. Description of the Prior Art
In this modern information based society, one of the major concerns is how to manage and store tremendous amounts of information. Compared to other kinds of storage media, the compact disk has a small size and a higher-density storage capacity. Due to developments in recordable and rewritable compact disk technology, consumers have the ability to utilize compact disk storage capacity on their personal computers.
In order to effectively manage the information stored on a compact disk, the data storage region of the compact disk is divided into many frames. Data can be stored in these frames according to a memory format. Each frame is identified by a minute/second, which means that a given frame corresponds to a particular time. The related time signal is known as the absolute time in pre-groove (ATIP).
A top view of a typical compact disk 10 is shown in FIG. 1. As is well known in the art, the compact disk 10 comprises a reflecting surface 13. A compact disk drive emits a laser beam onto the reflecting surface 13 of the compact disk 10, and the laser beam is reflected by different parts of the reflecting surface 13. The compact disk drive reads the information on the compact disk by collecting the reflected laser beam using an optical pickup.
On the reflecting surface 13 of the compact disk 10, there is a fine spiral track 11. Please refer to FIG. 1, which shows a magnified view 1A of the fine track 11. The track 11 is composed of two types of tracks, one being a data track 12 to record data, and the other being a wobble track 14 to record related time information of each frame. As illustrated in the magnified view 1A, the data track 12 has a continuously spiral shape, and the wobble track 14 has an oscillating shape. Additionally, the curvature of the wobble track 14 is composed of small segment curves with two different periods, D1 and D2.
In a further magnified view 1B in FIG. 1, an interrupt and discontinuity record mark 16 is shown within data track 12. The length of each record mark 16 varies, and the reflection characteristic of the record mark 16 is different from that of the reflecting surface 13. The record mark 16 is used to allow the compact disk drive to be able to write data onto the compact disk 10. The surface of the wobble track 14 protrudes beyond the reflecting surface 13. The data track 12 is located inside a groove formed by the raised wobble track 14 as is shown in FIG. 2, which is a three-dimensional perspective view of the magnified view 1B of the compact disk 10.
The process used to control the optical pick up in the compact disk drive to extract data from the wobble track 14 will now be explained using FIG. 3. As the compact disk rotates, an optical pick up 20 can be thought of as moving over the track 11 of the compact disk along the direction of arrow 18. In addition to a optical receiver (not shown) for reading the data from record mark 16 within the data track 12, there are four sensors within the optical pick up 20, namely Sa, Sb, Sc, and Sd. These four sensors are utilized to extract information from the wobble track 14. The positions of sensors Sa and Sd are controlled to be located within the groove of wobble track 14.The positions of sensors Sb and Sc are controlled to be located in the protruded area of the wobble track 14. The reflected laser beam intensities detected by the four sensors Sa, Sb, Sc, and Sd are different because of the difference in reflecting quality between the groove and the protruded area of the wobble track 14. As the optical pick up 20 moves along a straight path from the position shown to position P1, the sensing values of the four sensors Sa, Sb, Sc, and Sd change. A wobble signal can be generated by subtracting the electrical sensing value of Sa from that of Sd.
A waveform diagram of the wobble signal is shown in FIG. 4 with time along the abscissa and waveform amplitude along the ordinate. As described previously, the sensing values of the sensors Sa, Sb, Sc, and Sd change with time because the pick-up head 20 will detect different locations of the wobble track 14 when the compact disk 10 keeps rotating. This causes the wobble signal to change in amplitude with time. The curvature of the wobble track 14 is composed of two different curves with two different periods, D1 and D2. Consequently, the wobble signal waveform is also composed of two different curves with two different periods, T1 and T2, corresponding to the two periods, D1 and D2. Time information related to the control of the compact disk drive is stored by the changing period of the wobble track 14 and present in the wobble signal.
Waveform diagrams of the information associated with the wobble signal are shown in FIG. 5, which has time along the abscissa. FIG. 5 shows a wobble signal 22, an ATIP signal 24, a data clock signal 26, and a time data signal 28. After undergoing a waveform clipping process, the sinusoidal wobble signal in FIG. 4 is transformed into the square wave wobble signal 22. The integrity of the different periods, T1 and T2, is maintained in the new wobble signal 22. The portion of the wobble signal 22 with the period T1, and frequency 1/T1, corresponds to a high level signal in the ATIP signal 24. Likewise, portion of the wobble signal with the period T2, and frequency of 1/T2, corresponds to a low level signal in the ATIP signal 24. As a result, the time data corresponding to the record related area of the compact disk can be extracted from the wobble signal 22 using frequency demodulation.
The extraction of time data 28 is done using both the ATIP signal 24 and the data clock signal 26. As shown in FIG. 5, the data clock signal 26 is utilized to synchronize the reading of the ATIP signal 24.The ATIP signal 24 is read at each clock pulse in the clock signal 26 to generate the sequential bit sequence shown in the time data signal 28. A period TB of the data clock signal 26 defines the time duration of one bit in the ATIP signal 24. Through analysis of the time data 28, the information stored in the related records of the compact disk can be found and extracted. Also, when writing data to the compact disk, the data to be stored on the compact disk can be put into the correct record area.
The compact disk drive also utilizes a wobble clock to assist in the generation of the wobble signal. The wobble clock frequency is related to the average frequency of the changing frequencies, 1/T1 and 1/T2, in the wobble signal. The average frequency is close to (1/T1+1/T2)/2 with little deviation, and the frequency of wobble clock is normally twice as high as this average frequency.
A functional block diagram of a prior art data circuit 30 is shown in FIG. 6. The block diagram explains how a time data signal 50 and a wobble clock 48 are obtained from a wobble signal 32. Fundamentally, the prior art circuit 30 is very similar to a phase-locked loop (PLL). After the wobble signal 32 is determined, the wobble signal 32 is operated on by a pre-processing circuit 34, which is usually a frequency divider, and then fed to an input 36A of a phase comparator 36. The phase comparator 36 compares two input signals from two inputs, 36A and 36B, and outputs a corresponding signal to an output 36C according to the comparison result. The output 36C of the phase comparator 36 is connected to a low pass filter 40. The low pass filter 40 smoothes the signal from the phase comparator 36 and generates a control signal at node 38. As shown in FIG. 6, the control signal output at node 38 is provided to a wobble clock generator 46, a voltage controlled oscillator (VCO) 42, and a waveform shaping circuit 52. The wobble signal 32 contains two different frequencies, 1/T1 and 1/T2, and the control signal at node 38 reflects this. Specifically, the control signal changes with the changing frequency of the wobble signal 32, and forms a control waveform signal. The control waveform signal at node 38 is further processed by the waveform shaping circuit 52 and output as a time data signal 50. Similarly, the control waveform signal at node 38 is processed by the wobble clock generator 46 to create the wobble clock 48. In order for the circuit 30 to function like a PLL, the control voltage at node 38 is fed to a voltage controlled oscillator to generate a period signal. The period signal is further handled by a feedback processing circuit 44, which is functionally related to the pre-processing circuit 34, and then fed-back to the input 36B of the phase comparator 36 as a reference level for comparison. The reference level is utilized by the phase comparator 36 to distinguish between the different frequencies of the wobble signal 32.
The prior art circuit 30 has the major disadvantage of being designed using analog components. The charge pump in the phase comparator 36, the capacitors and resistors of the low pass filter 40 and the voltage controlled oscillator 42, are all analog components. Conversely, the data processing and signal controlling circuit modules in the compact disk drive are realized by programmable digital integrated circuits, such as digital signal processing chips. Combining analog and digital circuits is expensive and labor intensive.